Method for producing golf ball

ABSTRACT

A method of manufacturing golf balls composed of a core and a cover of one or more layer that encloses the core, at least one cover layer being made of a thermoplastic polyurethane composition characterized by including at least 90 wt % of a thermoplastic polyurethane obtained by a polyurethane-forming reaction of an organic diisocyanate compound with a long-chain polyol and a chain extender, which thermoplastic polyurethane composition cover layer has an inherent viscosity in a DMF solvent of larger than 1.5 dl/g and an inherent viscosity in a 0.05 mol/L n-butylamine-containing DMF solution of larger than 0.5 dl/g, the golf ball manufacturing method being characterized by: conditioning the thermoplastic polyurethane to a pre-processing moisture content of 500 ppm or less, then injection molding the moisture-conditioned thermoplastic polyurethane to form the thermoplastic polyurethane composition cover layer. Golf balls produced by this method have a high rebound and excellent spin characteristics and scuff resistance, are recyclable in a molding operation, and have excellent manufacturability.

BACKGROUND OF THE INVENTION

The present invention relates to a method of manufacturing golf balls inwhich a specific thermoplastic polyurethane composition is used in thecover. More specifically, the invention relates to a method ofmanufacturing golf balls in which a thermoplastic polyurethanecomposition that is recyclable in a molding operation is used in thecover, which golf balls have a high rebound, excellent spincharacteristics and scuff resistance, and an excellentmanufacturability.

The use of polyurethane materials as golf ball cover materials hasreceived some attention in recent years. Polyurethane materials arebroadly divided, based on the process used to make molded articlestherefrom, into thermoset polyurethane materials and thermoplasticpolyurethane materials. Molded articles made of thermoset polyurethanematerials can be obtained by mixing under applied heat a urethaneprepolymer having isocyanate end groups with a liquid starting materialsuch as a polyol or a polyamine as the curing agent, then pouring themixture directly into a mold and heating to effect a urethane curingreaction.

Numerous golf balls which use such a thermoset polyurethane materialhave been disclosed in the prior art (e.g., Patent Document 1: U.S. Pat.No. 5,334,673, Patent Document 2: U.S. Pat. No. 6,117,024, and PatentDocument 3: U.S. Pat. No. 6,190,268). Methods of molding thermosetpolyurethane materials are described in, for example, Patent Document 4:U.S. Pat. No. 5,006,297, Patent Document 5: U.S. Pat. No. 5,733,428,Patent Document 6: U.S. Pat. No. 5,888,437, Patent Document 7: U.S. Pat.No. 5,897,884, and Patent Document 8: U.S. Pat. No. 5,947,843.

Because moldings made of thermoset polyurethane materials lackplasticity when heated, the starting materials and molded articlescannot be recycled. Moreover, given the length of the heating and curingstep and of the cooling step and given the difficulty of controlling themolding time owing to the high reactivity under heating and theinstability of the starting materials, the manufacturability ofspecialty moldings such as golf ball covers (moldings which enclose acore material) made of thermoset polyurethane materials is regarded asinefficient.

By contrast, moldings made of thermoplastic polyurethane materials arenot obtained by directly reacting the starting materials. Instead, alinear polyurethane material synthesized using starting materials and aproduction method which differ somewhat from those for the thermosetpolyurethane materials described above is employed in the moldingoperation. Such a polyurethane material is thermoplastic, andthermoplasticized polyurethane materials are of a nature as to solidifyon cooling. Such polyurethane materials can thus be molded using aninjection molding machine. The injection molding of thermoplasticpolyurethane materials requires a much shorter molding time thanthermoset polyurethane materials and moreover is suitable for precisionmolding, making it ideal as a process for molding golf ball covers. Inaddition, thermoplastic polyurethane materials are recyclable and arefriendly to the global environment. Golf balls made using thermoplasticpolyurethane materials are disclosed in, for example, Patent Document 9:U.S. Pat. No. 3,395,109, Patent Document 10: U.S. Pat. No. 4,248,432 andPatent Document 11: U.S. Pat. No. 4,442,282.

However, prior-art golf ball covers made with thermoplastic polyurethanematerials have been unable to satisfy at the same time requirements forfeel on impact, controllability, rebound and scuff resistance when hitwith an iron.

To address this need, Patent Document 12: JP-A 9-271538 discloses a golfball cover made using a high-resilience thermoplastic polyurethanematerial. Yet, even this golf ball cover falls short in terms of itsscuff resistance when hit with an iron.

Patent Document 13: JP-A 11-178949 describes a golf ball cover which hasa relatively good scuff resistance when hit with an iron and is composedprimarily of the reaction product of a thermoplastic polyurethanematerial with an isocyanate compound. In this cover, an isocyanatecompound such as a diisocyanate or a block isocyanate dimer is added asan additive to the thermoplastic polyurethane material. Addition iscarried out during melt mixing under applied heat using an extruder orduring injection molding, with the reaction being effected duringmolding.

However, in the molding of a cover according to JP-A 11-178949 above,the isocyanate compound is hard to handle because it loses its activityin the presence of moisture, thus making it difficult to obtain a stablereaction product. In the case of blocked isocyanates, which are highlyresistant to moisture absorption, the blocking agent that dissociatesunder heating has a strong odor, making it unsuitable for use in moldingcovers. Moreover, when the isocyanate compound is in the form of apowder or a solution, it is difficult to control the amount of additionto the thermoplastic polyurethane material, making control of the golfball cover properties a challenge. Furthermore, owing to melting pointand melt viscosity differences between the thermoplastic polyurethanematerial and the isocyanate compound, slippage arises within the moldingmachine, which sometimes makes thorough kneading impossible to achieve.In this prior art, for the reasons given above, control of the effectsof moisture within the cover material and of the amount of additiveincluded therein has been inadequate, making it impossible to achievegolf ball covers which are fully satisfactory in terms of their scuffresistance-improving effects.

The preferred thermoplastic polyurethane material described in JP-A11-178949 is based on an aliphatic isocyanate. However, thisthermoplastic polyurethane material has a very large reactivity withisocyanate, making the reaction difficult to control. As a result, oneproblem is that gelation tends to arise before the mixture is used ininjection molding, making it impossible to ensure sufficient plasticity.Another problem is that gelation sometimes occurs during the moldingoperation. Yet another problem is that the resin being recycledsometimes gels, as a result of which reclamation becomes impossible.These problems have made it difficult to put the above technology topractical use.

Patent Document 14: JP-B 58-2063 (U.S. Pat. No. 4,347,338) describes amethod of manufacturing thermoset polyurethane molded articles whichinvolves intimately mixing a compound bearing two or more isocyanategroups with a thermoplastic resin which does not react with isocyanategroups, blending the resulting mixture with a thermoplastic polyurethanematerial, then furnishing the blend to a molding machine and molding.However, the objects of this prior art are simply to improve theresistance to solvents and to continuous and repeated friction; thepublished specification does not indicate the use of this prior-artmolding material as a golf ball cover material. Accordingly, therecontinues to exist a desire for a golf ball cover material which cansatisfy a number of properties required of golf balls; i.e., rebound,distance, spin characteristics, controllability, feel on impact, scuffresistance, cut resistance, and discoloration resistance.

Patent Document 15: JP-A 2002-336378 discloses a golf ball which uses acover stock composed of a thermoplastic polyurethane material and anisocyanate mixture. This cover stock is a thermoplastic polyurethanematerial which is recyclable and has a high rebound and an excellentscuff resistance. Although such cover stock exhibits both the goodmanufacturability of a thermoplastic polyurethane and physicalproperties on a par with those of thermoset polyurethanes, it also has anumber of drawbacks, including the formation of scorched material whenthe isocyanate mixture is charged into the molding machine and a poormolding stability.

SUMMARY OF THE INVENTION

It is thus an object of this invention to provide a method ofmanufacturing golf balls which have a high rebound, excellent spincharacteristics and scuff resistance, are recyclable in a moldingoperation, and have excellent manufacturability.

As a result of extensive investigations, we have found that the aboveproblems can be resolved by a method of manufacturing golf ballscomposed of a core and a cover of one or more layer that encloses thecore, at least one cover layer being made of a thermoplasticpolyurethane composition composed primarily of a thermoplasticpolyurethane obtained by a polyurethane-forming reaction among along-chain polyol, an organic diisocyanate compound and a chainextender, which an inherent viscosity in a DMF solvent of a cover layeris larger than 1.5 dl/g and an inherent viscosity in a 0.05 mol/Ln-butylamine-containing DMF solution of a cover layer is larger than 0.5dl/g, the golf ball manufacturing method being characterized byconditioning the thermoplastic polyurethane to a pre-processing moisturecontent of 500 ppm or less, then injection molding themoisture-conditioned thermoplastic polyurethane to form thethermoplastic polyurethane composition cover layer.

In the manufacturing method of the invention, the thermoplasticpolyurethane is conditioned to a pre-processing moisture content of 500ppm or less. When thermoplastic polyurethane is thermally processed(e.g., injection molded), to avoid undesirable effects such as foamingof the resin during the molding operation, it is desirable for thepolyurethane to be dried at 80 to 90° C. for 1 to 2 hours prior toprocessing (Poriuretan jushi [Polyurethane Resins] by Keiji IWATA,published by Nikkan Kogyo Shimbun, Ltd., p. 165). However, in light ofthe property requirements for golf ball covers, I have found frominvestigations that further limiting the moisture content within theabove range is desirable, particularly from the standpoint of stabilityduring injection molding and the resin properties after molding.

Similarly, in the inventive method of manufacture, it is also possibleto use recyclable material composed of the above thermoplasticpolyurethane composition, in which case the recyclable thermoplasticpolyurethane is conditioned to a pre-processing moisture content of5,000 ppm or less. Moreover, I have found that limiting thepre-processing moisture content of the recyclable polyurethane to theabove moisture content is desirable for suppressing a rise in viscosityduring molding.

Also, in the inventive method of manufacture, after covering the corewith the thermoplastic polyurethane composition, it is advantageous toadminister heat treatment for efficient hardness recovery and to promotecrosslinking. In the prior art, to achieve a good hardness recovery,heat treatment of the molded article has often been carried out usinghot-air treatment or oven treatment. In the present invention, when heattreatment is carried out on the above thermoplastic polyurethanecomposition, I have found that carrying out heat treatment in thepresence as well of moisture unexpectedly contributes to the formationof strong crosslinks, namely urea bonds and biuret bonds, and providesenhanced scuff resistance.

Accordingly, the invention provides the following golf balls.

-   Claim 1: A method of manufacturing golf balls composed of a core and    a cover of one or more layer that encloses the core, at least one    cover layer being made of a thermoplastic polyurethane composition    characterized by including at least 90 wt % of a thermoplastic    polyurethane obtained by a polyurethane-forming reaction of an    organic diisocyanate compound with a long-chain polyol and a chain    extender, which thermoplastic polyurethane composition cover layer    has an inherent viscosity in a DMF solvent larger than 1.5 dl/g and    an inherent viscosity in a 0.05 mol/L n-butylamine-containing DMF    solution of larger than 0.5 dl/g, the golf ball manufacturing method    being characterized by: conditioning the thermoplastic polyurethane    to a pre-processing moisture content of 500 ppm or less, then    injection molding the moisture-conditioned thermoplastic    polyurethane to form the thermoplastic polyurethane composition    cover layer.-   Claim 2: The manufacturing method of claim 1 which is characterized    in that the long-chain polyol is a polyether polyol having a    number-average molecular weight of 1,500 to 5,000.-   Claim 3: The manufacturing method of claim 1, wherein the    thermoplastic polyurethane composition is covered over the core,    then heat treated at a temperature of at least 50° C.-   Claim 4: The manufacturing method of claim 3, wherein heat treatment    is carried out at a relative humidity of at least 50%.-   Claim 5: The manufacturing method of claim 3, wherein heat treatment    is carried out at a relative humidity of less than 50%.-   Claim 6: The manufacturing method of claim 3, wherein heat treatment    is carried out in a liquid.-   Claim 7: A method of manufacturing golf balls composed of a core and    a cover of one or more layer that encloses the core, at least one    cover layer being made of a thermoplastic polyurethane composition    characterized by including at least 90 wt % of a thermoplastic    polyurethane obtained by a polyurethane-forming reaction between a    long-chain polyol, an organic diisocyanate compound and a chain    extender, which thermoplastic polyurethane composition cover layer    has an inherent viscosity in a DMF solvent of larger than 1.5 dl/g    and an inherent viscosity in a 0.05 mol/L n-butylamine-containing    DMF solution of larger than 0.5 dl/g, the golf ball manufacturing    method being characterized in that the thermoplastic polyurethane    composition cover layer is composed of two kinds of thermoplastic    polyurethane or polyurethane composition A and B which satisfy the    condition that the inherent viscosity of “B” in DMF is larger than    the inherent viscosity of “A” in DMF and the condition that the    inherent viscosity of “B” in 0.05 mol/L n-butylamine-containing DMF    solution is larger than the inherent viscosity of “A” in 0.05 mol/L    n-butylamine-containing DMF and which have been blended in    proportions that satisfy the following conditions:    A1<500 ppm    B1<5,000 ppm    100/1≧A2/B2≧100/150    (A1×A2+B1×B2)/(A2+B2)≦3,000 ppm    (wherein A1 is the moisture content in thermoplastic polyurethane or    polyurethane composition A, A2 is the weight of thermoplastic    polyurethane or polyurethane composition A, B1 is the moisture    content in thermoplastic polyurethane or polyurethane composition B,    and B2 is the weight of thermoplastic polyurethane or polyurethane    composition B).-   Claim 8: The golf ball manufacturing method of claim 7 which is    characterized in that B is recycled A.

DETAILED DESCRIPTION OF THE INVENTION

The invention is described more fully below.

The inventive method of manufacturing golf balls composed of a core anda cover of one or more layer that encloses the core, at least one coverlayer being made of a thermoplastic polyurethane compositioncharacterized by including at least 90 wt % of a thermoplasticpolyurethane obtained by a polyurethane-forming reaction among along-chain polyol, an organic diisocyanate compound and a chainextender, which thermoplastic polyurethane composition cover layer hasan inherent viscosity in a DMF solvent of larger than 1.5 dl/g and aninherent viscosity in a 0.05 mol/L n-butylamine-containing DMF solutionof larger than 0.5 dl/g, is characterized by conditioning thethermoplastic polyurethane to a pre-processing moisture content of 500ppm or less, then injection molding the moisture-conditionedthermoplastic polyurethane to form the thermoplastic polyurethanecomposition cover layer.

The thermoplastic polyurethane in the invention has a structure whichincludes soft segments made of a polymeric polyol that is a long-chainpolyol (polymeric glycol), and hard segments made of a chain extenderand an organic diisocyanate. Here, the polymeric polyol used as astarting material is not subject to any particular limitation, and maybe any that has been used in the prior art relating to thermoplasticpolyurethanes. Examples include polyester polyols and polyether polyols,although polyether polyols are preferable to polyester polyols becausethey enable the synthesis of thermoplastic polyurethanes having a highrebound resilience and excellent low-temperature properties.

Illustrative examples of polyether polyols include polytetramethyleneglycol and polypropylene glycol, although polytetramethylene glycol isespecially preferred in terms of the rebound resilience andlow-temperature properties. The polyether polyol has a number-averagemolecular weight of preferably 1,500 to 5,000, with a molecular weightof 2,000 to 4,000 being especially preferred for the synthesis ofthermoplastic polyurethane having a high rebound resilience.

In this specification, “number-average molecular weight” refers to thenumber-average molecular weight computed based on the hydroxyl numbermeasured in accordance with JIS K-1557.

Chain extenders suitable for use include those used in the prior artrelating to thermoplastic polyurethanes. For example,low-molecular-weight compounds which have a molecular weight of not morethan 400 and bear on the molecule two or more active hydrogen atomscapable of reacting with isocyanate groups are preferred. Illustrative,non-limiting, examples of the chain extender include 1,4-butyleneglycol, 1,2-ethylene glycol, 1,3-butanediol, 1,6-hexanediol and2,2-dimethyl-1,3-propanediol. Of these chain extenders, aliphatic diolshaving 2 to 12 carbons are preferred, and 1,4-butylene glycol isespecially preferred.

Suitable organic diisocyanates include those used in the prior artrelating to thermoplastic polyurethanes. Illustrative, non-limiting,examples include aromatic diisocyanates such as 4,4′-diphenylmethanediisocyanate, 2,4-toluene diisocyanate and 2,6-toluene diisocyanate; andaliphatic diisocyanates such as hexamethylene diisocyanate. Depending onthe type of isocyanate used, the crosslinking reaction during injectionmolding may be difficult to control. In the practice of the invention,to provide a balance between stability at the time of production and theproperties that are manifested, it is most preferable to use4,4′-diphenylmethane diisocyanate, which is an aromatic diisocyanate.

The thermoplastic polyurethane in the invention is most preferably onesynthesized using a polyether polyol and an aromatic diisocyanate, andspecifically one synthesized using polytetramethylene glycol having anumber-average molecular weight of at least 2,000 as the polyetherpolyol and 4,4′-diphenylmethane diisocyanate as the aromaticdiisocyanate.

The above thermoplastic polyurethane is prepared by apolyurethane-forming reaction of the organic diisocyanate compound withthe long-chain polyol and the chain extender. By setting the content ofnitrogen atoms in the thermoplastic polyurethane that originate from theorganic diisocyanate compound, as a percentage of the combined weight ofthe long-chain polyol, organic diisocyanate compound and chain extender,within a range of 4.0 to 6.5 wt %, golf balls in which theabove-mentioned properties such as rebound, spin characteristics, scuffresistance and manufacturability are even better can be obtained.

The ratio of active hydrogen atoms to isocyanate groups in thepolyurethane-forming reaction can be set within a desirable range so asto enable golf balls made of a thermoplastic polyurethane compositionthat are endowed with even better properties such as rebound, spincharacteristics, scuff resistance and manufacturability to be obtained.

No particular limitation is imposed on the method of preparing thethermoplastic polyurethane. Production may be carried out by either aprepolymer process or a one-shot process in which the long-chain polyol,chain extender and organic diisocyanate compound are used and a knownurethane-forming reaction is carried out. Of these, a process in whichmelt polymerization is carried out in a substantially solvent-free stateis preferred. Production by continuous melt polymerization using amultiple screw extruder is especially preferred.

The thermoplastic polyurethane is included as a major component in thethermoplastic polyurethane composition used in the cover of theinventive golf ball. Here, “major component” signifies that thethermoplastic polyurethane accounts for at least 90 wt %, preferably atleast 95 wt %, more preferably at least 99 wt %, and up to 100 wt %, ofthe thermoplastic polyurethane composition.

In addition to the above thermoplastic polyurethane, the thermoplasticpolyurethane composition may also include other ingredients. Examples ofsuch other ingredients include thermoplastic polymers other thanthermoplastic polyurethane, such as polyester elastomers, polyamideelastomers, ionomer resins, styrene block elastomers, polyethylene andnylon resins. The amount in which such thermoplastic polymers other thanthermoplastic polyurethane are included, per 100 parts by weight of thethermoplastic polyurethane serving as the essential component therein,is generally 0 to 10 parts by weight, preferably 0 to 5 parts by weight,and more preferably 0 to 1 part by weight. This amount may be selectedas appropriate for such purposes as adjusting the hardness, improvingthe resilience, enhancing the flow properties, and improving theadhesion of the cover stock.

If necessary, the cover stock may include also various additives otherthan the ingredients making up the above thermoplastic polyurethane. Forexample, additives such as pigments, dispersants, antioxidants, lightstabilizers, ultraviolet absorbers and parting agents may be suitablyincluded.

In the practice of the invention, the method of forming the cover layermay involve, for example, feeding the above cover stock to an injectionmolding machine and injecting the molten cover stock around the core soas to form the cover layer. The molding temperature varies with the typeof thermoplastic polyurethane, but is generally in a range of 150 to250° C.

If injection molding is carried out, it is desirable, though notessential, to carry out a nitrogen purge or vacuum treatment at some orall places on the resin paths from the resin feed area to the moldinterior, and thus carry out molding in a low-humidity environment.

Here, in the practice of the invention, when the cover layer is formed,the thermoplastic polyurethane composition or thermoplastic polyurethaneto be used as the cover material has a pre-processing moisture contentof generally 500 ppm or less, preferably 300 ppm or less, and morepreferably 100 ppm or less. A pre-processing moisture content within thethermoplastic polyurethane composition or thermoplastic polyurethane inexcess of 500 ppm may result in undesirable effects such as foamingduring the molding operation and in a rise in the resin viscosity, andmay also be accompanied by declines in the physical properties in viewof property requirements for golf ball covers.

A thermoplastic polyurethane composition or thermoplastic polyurethaneconditioned to a moisture content of 500 ppm or less can be obtained byusing equipment effective for moisture removal, such as a vacuum drier,a hot-air drier or a dehumidifying dryer to carry out treatment,preferably within a temperature range of 40 to 150° C. for a period of 1to 10 hours. Treatment in a dried atmosphere of air or nitrogen, forexample, having a dew point of −20° C. or below is more preferred.

A commercial product may be used for this purpose, provided it is athermoplastic polyurethane composition or thermoplastic polyurethaneconditioned to a moisture content of 500 ppm or less.

In the practice of the invention, when the cover layer is molded, it isalso advantageous to suitably pulverize runners and the like that formduring injection molding (here, “runners” refers to excess resin thathas solidified in the mold channels for uniformly feeding molten resinduring injection molding) to form a recyclable material, and eitherreuse this recyclable material as the cover stock or blend it withthermoplastic polyurethane composition composed primarily of newlysynthesized thermoplastic polyurethane.

Here, when the above recyclable material is used as part of the coverstock, the pre-processing moisture content of the recyclable material isgenerally 5,000 ppm or less, and preferably 3,000 ppm or less. If thepre-processing moisture content of the recyclable material exceeds 5,000ppm, the result may be undesirable effects such as foaming duringmolding, an increase in the viscosity of the resin, and a decline in thephysical properties of the resulting cover.

Moreover, when this recyclable material is blended with a thermoplasticpolyurethane composition composed primarily of newly synthesizedthermoplastic polyurethane, the blending ratio, expressed as the weightratio (thermoplastic polyurethane composition): (recyclable material),is typically from 100:1 to 100:150, and preferably from 100:100 to100:25. If the proportion of recyclable material is too high, themoldability may worsen due to an increase in viscosity and the physicalproperties of the golf ball cover may decline.

After the cover stock has been molded as described above, its propertiesas a golf ball cover can be further improved by carrying out annealingso as to induce crosslinking reactions to proceed even further.“Annealing,” as used herein, refers to aging the cover layer in a fixedenvironment for a fixed length of time.

The crosslinking reactions are believed to involve the reaction ofremaining isocyanate groups with remaining hydroxyl groups in thethermoplastic polyurethane composition to form new urethane bonds, andaddition reactions by remaining isocyanate groups with the urethanegroups on the thermoplastic polyurethane to form allophanate or biuretcrosslinks.

The annealing temperature can be set to generally at least 40° C.,preferably at least 45° C., more preferably at least 50° C., and evenmore preferably at least 70° C. If the temperature during annealing istoo low, this step may do little to induce the crosslinking reactions toproceed. On the other hand, in cases where the cover is composed of twoor more layers and these cover layers include a layer formed of anionomer resin, or in cases where the inventive golf ball is composed ofa core, an intermediate layer enclosing the core, and a cover enclosingthe intermediate layer, with the intermediate layer being made of anionomer resin, if annealing is carried out at too high a temperature,the temperature may exceed the cluster melting point Ti of the ionomerresin, as a result of which the rebound of the golf ball may decrease.Also, exceeding the melting point Tm of the ionomer resin may result indeformation of the intermediate layer.

The relative humidity at the time of annealing is generally at least50%, and preferably at least 60%, although it may be less than 50%. Toform urea bonds and biuret bonds as strong crosslinks and therebyenhance the scuff resistance, a relative humidity of at least 50% ispreferred. To suppress the bleedout of compounding chemicals and avoidan adverse impact on printability, a relative humidity of less than 50%is preferred.

No particular limitation is imposed on the means for carrying out suchannealing. Annealing may be carried out in an oven, it may be carriedout by installing a heat source place within the process and having theworkpieces pass over that place, it may be carried out within aconstant-temperature, constant-humidity chamber or an apparatus having asimilar function, or it may be carried out within a liquid. Of these,from the standpoint of the ease of installing a heat source environmentand the ability to selectively and efficiently form urea bonds andbiuret bonds, it is preferable to carry out annealing within a liquid.

The annealing time may be set as appropriate for the annealingtemperature, within a range that achieves the desired treatment effects.The annealing time is generally at least 30 minutes, preferably at least1 hour, and most preferably at least 2 hours.

At least one of the one or more layers in the cover of the inventivegolf ball is made of the above-described thermoplastic polyurethanecomposition. The cover layer made of this thermoplastic polyurethanecomposition has a surface hardness, expressed as the durometer Dhardness, of generally 30 to 90, preferably 35 to 85, more preferably 40to 80, and even more preferably 45 to 75. If the surface hardness of thecover layer is too low, the spin rate when the ball is hit with a drivermay increase, shortening the carry of the ball. On the other hand, ifthe surface hardness of the cover layer is too high, the feel of theball on impact may worsen and the urethane material may have a poorresilience and durability.

“Durometer D hardness” refers herein to the hardness measured with atype D durometer in accordance with JIS K7215.

The above-described cover layer has a rebound resilience of generally atleast 35%, preferably at least 40%, more preferably at least 45%, andeven more preferably at least 47%. Because thermoplastic polyurethanedoes not inherently have that good a resilience, it is desirable toscrupulously select the rebound resilience. If the rebound resilience ofthe cover layer is too low, the distance traveled by the golf ball maydramatically decrease. On the other hand, if the rebound resilience ofthe cover layer is too high, the initial velocity on shots of under 100yards that require control and on putts may be too high and the feel ofthe ball when played may not agree with the golfer.

“Rebound resilience” refers herein to the rebound resilience obtained inaccordance with JIS K7311.

The core in golf balls made by the inventive method is not subject toany particular limitation. For example, various cores that may be usedinclude solid cores for two-pieces balls, solid cores having a pluralityof vulcanized rubber layers, solid cores having a plurality of resinlayers, and thread-wound cores having a rubber thread layer. Noparticular limitation is imposed on the diameter, weight, hardness,materials and other characteristics of the core.

In cases where golf balls made by the method of the invention have aconstruction that includes an intermediate layer, no particularlimitation is imposed on the hardness, materials, thickness and othercharacteristics of the intermediate layer. If necessary, a primer layermay be provided to improve adhesion between the intermediate layer andthe cover.

It is preferable for the cover layer to have a thickness within a rangeof 0.1 to 5.0 mm. The cover layer is not limited to a single layer, andmay be formed with a multilayer construction. If the cover is formedwith a multilayer construction, the overall thickness of the cover maybe set within the foregoing range.

Golf balls obtained by the manufacturing method of the invention arepreferably formed to a diameter and weight in accordance with the Rulesof Golf, and are generally formed to a diameter of not less than 42.67mm and a weight of not more than 45.93 g. The diameter is preferablyfrom 42.67 to 42.9 mm. It is appropriate for deflection by the ball whensubjected to a load of 980 N (100 kg) to be generally from 2.0 to 4.0mm, and especially from 2.2 to 3.8 mm.

The golf ball manufacturing method of the invention is suitable forproducing golf balls which have a high rebound, excellent spincharacteristics and scuff resistance, are recyclable in a moldingoperation, and have an excellent manufacturability.

EXAMPLES

The following examples of the invention and comparative examples areprovided by way of illustration and not by way of limitation.

(1) Melt Viscosity:

The melt viscosity of thermoplastic polyurethane that had beenvacuum-dried under a pressure of 1,333.3 Pa (10 torr) or less at 120° C.for 1 hour was measured using a CFT-500D capillary rheometermanufactured by Shimadzu Corporation (nozzle dimensions: 1 mmdiameter×10 mm length) under a load of 490.3 N (50 kgf) and at atemperature of 220° C.

Examples 1 to 10, and Comparative Examples 1 to 3

Core Formulation Polybutadiene 100 parts by weight Zinc acrylate 24.5parts by weight Zinc oxide 12 parts by weight Dicumyl peroxide 1 part byweight Zinc salt of pentachlorothiophenol 1 part by weight

The core material of the above formulation was kneaded, following whichit was molded and vulcanized at 155° C. for 20 minutes, therebyobtaining a 38.5 mm diameter solid core for a two-piece solid golf ball.The polybutadiene rubber used was BROL produced by JSR Corporation. Theresulting core had a specific gravity of 1.17 g/cm³, a deflection of 3.4mm under a load of 980 N (100 kg), and an initial velocity, measured inaccordance with the USGA (R&A) measurement method, of 78.1 m/s.

The starting materials shown in Tables 1 to 3 (units: parts by weight)were worked at a temperature of 190° C. in a twin-screw extruder,thereby giving a cover stock.

The solid core was placed within a mold for injection molding and thecover stock was injection molded around the core, thereby givingtwo-piece golf balls in the respective examples of the invention andcomparative examples, each having a 2.1 mm thick cover. Followinginjection molding of the cover stock, annealing treatment wasadministered under the respective conditions shown in the tables. Theannealing time was 120 minutes. The resulting golf ball was held at roomtemperature for one week, following which the ball properties wereevaluated. Measurement of the physical properties of the cover wascarried out on samples prepared by injection molding a 2 mm thick sheet,subjecting the molded sheet to annealing treatment in the same way asdescribed above, and holding the annealed sheet at room temperature forone week. The recyclability and manufacturability of the cover stockwere also evaluated. The results are shown in Tables 1 to 3.

TABLE 1 Example 1 2 3 4 5 Thermoplastic Polyurethane 1 100 polyurethanePolyurethane 2 100 100 100 100 (parts by weight) Polyurethane 3Polyurethane 2R Titanium oxide (pbw) 3 3 3 3 3 Polyethylene wax (pbw) 11 1 1 1 Annealing treatment 50° C. oven ◯ ◯ 70° C. oven ◯ 50° C., 70% RH◯ 50° C., 30% RH ◯ 50° C. water Room temperature, in air Coverproperties Surface hardness (D hardness) 60 63 63 63 63 Reboundresilience (%) 50 51 51 51 51 Ball properties Diameter (mm) 42.7 42.742.7 42.7 42.7 Weight (g) 45.6 45.6 45.6 45.6 45.6 Hardness (mm) 2.9 2.82.8 2.8 2.8 Initial velocity (m/s) 76.9 76.8 76.1 76.8 76.8 Scuff 23° C.5 4 5 5 5 resistance 13° C. 4 4 5 4 4  0° C. 4 4 4 4 4 Recyclability yesyes yes yes yes Manufacturability good good good good good

TABLE 2 Example 6 7 8 9 10 Thermoplastic Polyurethane 1 100 polyurethanePolyurethane 2 100 100 (parts by weight) Polyurethane 3 100 100Polyurethane 2R 50 Titanium oxide (pbw) 3 3 3 3 3 Polyethylene wax (pbw)1 1 1 1 1 Annealing treatment 50° C. oven ◯ 70° C. oven 50° C., 70% RH50° C., 30% RH 50° C. water ◯ Room temperature, in air ◯ ◯ ◯ Coverproperties Surface hardness (D hardness) 64 63 60 63 64 Reboundresilience (%) 51 51 50 51 51 Ball properties Diameter (mm) 42.7 42.742.7 42.7 42.7 Weight (g) 45.6 45.6 45.6 45.6 45.6 Hardness (mm) 2.8 2.82.9 2.8 2.8 Initial velocity (m/s) 76.8 76.1 76.9 76.8 76.8 Scuff 23° C.5 4 4 4 4 resistance 13° C. 5 3 4 4 4  0° C. 4 3 4 3 3 Recyclability yesyes yes yes yes Manufacturability good good good good good

TABLE 3 Comparative Example 1 2 3 Thermoplastic polyurethanePolyurethane 4 50 50 50 (parts by weight) Polyurethane 5 50 50 50Titanium oxide (pbw) 3 3 3 Polyethylene wax (pbw) 1 1 1 Isocyanate (pbw)20 20 20 Annealing treatment 50° C. oven ◯ 70° C. oven ◯ 50° C., 70% RH50° C., 30% RH 50° C. water Room temperature, ◯ in air Cover Surfacehardness (D hardness) 62 62 62 properties Rebound resilience (%) 45 4545 Ball Diameter (mm) 42.7 42.7 42.7 properties Weight (g) 45.7 45.745.7 Hardness (mm) 2.8 2.8 2.8 Initial velocity (m/s) 76.8 76.8 76.0Scuff 23° C. 4 4 5 resistance 13° C. 4 4 4  0° C. 3 3 4 Recyclabilityyes yes yes Manufacturability NG NG NGPolyurethane 1

Kuramiron 5D51-W21-XWF0 (produced by Kuraray Co., Ltd.): a4,4′-diphenylmethane diisocyanate/1,4-butyleneglycol/poly(tetramethylene glycol) (abbreviated below as “MDI-BD-PTMG”)type thermoplastic polyurethane. Durometer D hardness, 51. Reboundresilience, 50%. Melt viscosity, 83 Pa·s.

Inherent viscosity in DMF solvent of polyurethane after molding, ≧2.0dl/g. Inherent viscosity in 0.05 mol/L n-butylamine-containing DMFsolution of polyurethane after molding, 0.8 dl/g.

Pre-processing moisture content, 40 ppm.

Polyurethane 2

Kuramiron 5D54-W21-XWF1 (produced by Kuraray Co., Ltd.): a MDI-BD-PTMGtype thermoplastic polyurethane. Durometer D hardness, 54. Reboundresilience, 51%. Melt viscosity, 75 Pa·s.

Inherent viscosity in DMF solvent of polyurethane after molding, ≧2.0dl/g. Inherent viscosity in 0.05 mol/L n-butylamine-containing DMFsolution of polyurethane after molding, 0.9 dl/g.

Pre-processing moisture content, 35 ppm.

Polyurethane 2R

Recyclable material from above Polyurethane 2. Pre-processing moisturecontent, 3,000 ppm.

Polyurethane 3

Kuramiron 5D54-W21-FWX3 (produced by Kuraray Co., Ltd.): a MDI-BD-PTMGtype thermoplastic polyurethane. Durometer D hardness, 54. Reboundresilience, 51%. Melt viscosity, 40 Pa·s.

Inherent viscosity in DMF solvent of polyurethane after molding, ≧2.0dl/g. Inherent viscosity in 0.05 mol/L n-butylamine-containing DMFsolution of polyurethane after molding, 1.0 dl/g.

Pre-processing moisture content, 30 ppm.

Polyurethane 4

Pandex T8295 (produced by DIC Bayer Polymer, Ltd.): a MDI-PTMG typethermoplastic polyurethane. JIS-A hardness, 97. Rebound resilience, 44%.Inherent viscosity in DMF solvent of polyurethane after molding, 0.6dl/g. Inherent viscosity in 0.05 mol/L n-butylamine-containing DMFsolution of polyurethane after molding, 0.6 dl/g.

Pre-processing moisture content, 300 ppm.

Polyurethane 5

Pandex T8260 (produced by DIC Bayer Polymer, Ltd.): a MDI-PTMG typethermoplastic polyurethane. Durometer D hardness, 56. Reboundresilience, 46%. Inherent viscosity in DMF solvent of polyurethane aftermolding, 0.6 dl/g. Inherent viscosity in 0.05 mol/Ln-butylamine-containing DMF solution of polyurethane after molding, 0.6dl/g.

Pre-processing moisture content, 250 ppm.

Polyethylene Wax

Sanwax 161P (produced by Sanyo Chemical Industries, Ltd.)

Isocyanate

Crossnate EM-30 (an isocyanate master batch produced by Dainichi SeikaColour & Chemicals Mfg. Co., Ltd.)

Annealing Treatment

The 50° C., 70% RH and 50° C., 30% RH conditions were arrived at using aconstant-temperature, constant-humidity chamber.

[Cover Properties]

Surface Hardness

The durometer D hardness was measured in accordance with JIS-K7215.

Rebound Resilience

The rebound resilience was measured in accordance with JIS-K7311.

[Ball Properties]

Hardness (mm)

The deflection when subjected to a load of 980 N (100 kg) was measured.

Initial Velocity (m/s)

Measured in accordance with the USGA (R&A) measurement method.

Scuff Resistance

The ball was held at respective temperatures of 23° C., 13° C. and 0° C.Using a swing robot machine, each ball was hit with a pitching wedge asthe club at a head speed of 33 m/s, after which damage from the impactwas visually rated according to the following criteria.

-   -   5: No damage at all or substantially free of apparent damage.    -   4: Slight damage observed, but of a substantially negligible        degree.    -   3: Surface somewhat frayed.    -   2: Surface frayed and portions of dimples missing.    -   1: Some dimples completely obliterated.

Recyclability

The runners that formed during injection molding were pulverized andevaluated for reusability. Here, “runners” refers to excess resin thathas solidified in the mold channels for uniformly feeding molten resinduring injection molding. The runners formed in the production of moldedarticles made of thermoplastic resin are generally pulverized, mixedwith resin, and reused.

-   -   Yes: Problems such as eccentricity did not arise even when up to        50% of pulverized runner resin was mixed with virgin resin and        molded.    -   No: The runner resin gelled and subsequently did not melt when        heated. In this state, reuse in molding was not possible.

Manufacturability

-   -   Good: Molding conditions during mass production were stable;        problems such as resin scorching rarely occurred.    -   NG: Molding conditions during mass production were unstable;        problems such as resin scorching frequently occurred.

1. A method of manufacturing golf balls composed of a core and a coverof one or more layer that encloses the core, at least one cover layerbeing made of a thermoplastic polyurethane composition characterized byincluding at least 90 wt % of a thermoplastic polyurethane obtained by apolyurethane-forming reaction of an organic diisocyanate compound with along-chain polyol and a chain extender, which thermoplastic polyurethanecomposition cover layer has an inherent viscosity in a DMF solvent oflarger than 1.5 dl/g and an inherent viscosity in a 0.05 mol/Ln-butylamine containing DMF solution of larger than 0.5 dl/g, the golfball manufacturing method being characterized by: conditioning thethermoplastic polyurethane, to a pre-processing moisture content of 500ppm or less, then injection molding the moisture-conditionedthermoplastic polyurethane to form the thermoplastic polyurethanecomposition cover layer.
 2. The manufacturing method of claim 1 which ischaracterized in that the long-chain polyol is a polyether polyol havinga number-average molecular weight of 1,500 to 5,000.
 3. Themanufacturing method of claim 1, wherein the thermoplastic polyurethanecomposition is covered over the core, then heat treated at a temperatureof at least 50° C.
 4. The manufacturing method of claim 3, wherein theheat treatment is carried out at a relative humidity of at least 50%. 5.The manufacturing method of claim 3, wherein the heat treatment iscarried out at a relative humidity of less than 50%.
 6. Themanufacturing method of claim 3, wherein the heat treatment is carriedout in a liquid.